Watch a blast of sound turn floating drops into bubbles

Burst a soap bubble and it will split into hundreds of droplets. Now we can do the reverse, turning droplets into bubbles, with a blast of sound.

Researchers have already shown it is possible to suspend small particles, such as droplets, in the air by counteracting gravity with pressure from ultrasound waves.

Duyang Zang at Northwestern Polytechnical University, China, and his colleagues have now discovered that higher sound pressure can create a bubble from a levitating droplet.

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The team placed a drop of soap solution between an acoustic levitator, a pair of emitting and reflecting panels that pump out sound. They increased the pressure on the droplet by gradually turning up the volume, pushing it into a flat film and then curving it into a bowl that filled with air.

Once the air in the bowl reached a certain volume, the trapped air molecules began to resonate with the sound waves and vibrate violently. This increased the pressure on the film, inflating it and eventually creating a bubble.

The team explored two alternative ways to manipulate the film shape: pulling the centre with a needle and passing a ring around the edge of the film. Both attempts were successful in producing bubbles.

The work is more than just a new way to blow bubbles, says Zang – the technique could create hollow-structured materials that float or can be used as capsules for medicine.